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RFC 0727

Telnet logout option

Pages: 3
Proposed STD

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Network Working Group                                       Mark Crispin
Request for Comments 727                                          MIT-AI
NIC 40025                                                  27 April 1977

                          TELNET Logout Option
1.  Command name and code.

   LOGOUT               18

2.  Command meanings.


      The sender of this command REQUESTS permission to, or confirms
      that it will, forcibly log off the user process at its end.


      The sender of this command REFUSES to forcibly log off the user
      process at its end.


      The sender of this command REQUESTS that the receiver forcibly log
      off the user process at the receiver's end, or confirms that the
      receiver has its permission to do so.


      The sender of this command DEMANDS that the receiver not forcibly
      log off the user process at the receiver's end.

3.  Default.



   i.e., no forcible logging off of the server's user process.

4.  Motivation for the option.

   Often, a runaway user process could be hung in such a state that it
   cannot be interrupted by normal means.  Conversely, the system itself
   could be bottlenecked so that response delays are intolerable.  A
   user (human or otherwise) eventually will time out out of frustration
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   and take the drastic means of closing the connection to free itself
   from the hung process.  In some situations, even the simple operation
   of logging out can take a long time.

   Some systems treat a close to mean that it should log out its user
   process under it.  However, many hosts merely "detach" the process so
   that an accidental close due to a user or temporary hardware error
   will not cause all work done on that job to be lost; when the
   connection is re-established, the user may "attach" back to its
   process.  While this protection is often valuable, if the user is
   giving up completely on the host, it can cause this hung job to
   continue to load the system.

   This option allows a process to instruct the server that the user
   process at the server's end should be forcibly logged out instead of
   detached.  A secondary usage of this option might be for a server to
   warn of impending auto-logout of its user process due to inactivity.

5.  Description of the option.

   When a user decides that it no longer wants its process on the server
   host and decides that it does not want to wait until the host's
   normal log out protocol has been gone through, it sends IAC DO
   LOGOUT.  The receiver of the command may respond with IAC WILL
   LOGOUT, in which case it will then forcibly log off the user process
   at its end.  If it responds with IAC WON'T LOGOUT, then it indicates
   that it has not logged off the user process at its end, and if the
   connection is broken, the process very possibly will be detached.

   A truly impatient user that feels that it must break away from the
   server immediately could even send IAC DO LOGOUT and then close. At
   the worst, the server would only ignore the request and detach the
   user process.  A server that implements the LOGOUT option should know
   to log out the user process despite the sudden close and even an
   inability to confirm the LOGOUT request!

6.  A sample implementation of the option.

   The server implements the LOGOUT option both for accepting LOGOUT
   requests and for auto-logout warning.

   Case 1:

      The user connects to the server, and starts interacting with the
      server.  For some reason, the user wishes to terminate interaction
      with the server, and is reluctant to go through the normal log out
      procedure, or perhaps the user is unable to go through the normal
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      log out procedure.  It does not want the process at the server any
      more, so it sends IAC DO LOGOUT.  The server verifies the request
      with IAC WILL LOGOUT, and then forcibly logs off the user process
      (perhaps by using a system call that causes another process to be
      logged out).  It does not have to close the connection unless the
      user closes or it wants to close.  Neither does it wait until the
      user has received its confirmation--it starts the log out
      immediately so if the user has in the mean time closed the
      connection without waiting for confirmation, its logout request
      still is performed.

   Case 2:

      The user connects to the server, and after logging in, is idle for
      a while, long enough to approach the server's autologout time.
      The server shortly before the autologout sends IAC WILL LOGOUT;
      the user sees this and sends IAC DON'T LOGOUT, and continues work
      on the host.  Nothing prevents the server from logging out the
      user process if inactivity continues; this can be used to prevent
      a malicious user from locking up a process on the server host by
      the simple expedient of sending IAC DON'T LOGOUT every time it
      sees IAC WILL LOGOUT but doing nothing else.